University of Oulu

Shu, Q., Klug, J. L., Medeiros, S. L. S., Heck, N. C., Liu, Y. 2020 Crystallization control for fluorine-free mold fluxes : effect of Na₂O content on non-isothermal melt crystallization kinetics, ISIJ International, Volume 60, Issue 11, Pages 2425-2435,

Crystallization control for fluorine-free mold fluxes : effect of Na₂O content on non-isothermal melt crystallization kinetics

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Author: Shu, Qifeng1,2; Klug, Jeferson Leandro3; Medeiros, Samuel Lucas Santos3;
Organizations: 1School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083 China
2Research Unit of Process Metallurgy, Oulu University, Oulu, FI-90014 Finland
3Post-GraduatiProgram in Materials Science and Engineering, Federal University of Ceará, Fortaleza, CEP 60440-554 Brazil
4Post-Graduation Program in Mining, Metallurgical and Materials Engineering, Federal University of Rio Grande do Sul, Porto Alegre, 91501-970 Brazil
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 1.8 MB)
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Language: English
Published: Iron and Steel Institute of Japan, 2020
Publish Date: 2021-01-22


There are increasing demands for developing fluorine-free mold fluxes for continuous casting of steel. When removing fluorine from mold flux composition, it is necessary to replace it with oxides, which must maintain the technological parameters, related to viscosity, melting characteristics, and crystallization behavior. For industrial developments in the CaO–SiO₂–Na₂O–Al₂O₃–TiO₂–B₂O₃–MgO (with basicity = 1, Al₂O3 = 7%, TiO₂ = 5%, B₂O₃ = 3%, MgO = 2%) slag system, it is necessary to know the effect of Na₂O concentration regarding crystallization kinetics. This is especially important for fluorine-free mold fluxes for peritectic steel slab casting. In this work, the crystals’ precipitation sequence for this system during cooling was determined, combining Differential Scanning Calorimetry (DSC), X-Ray Diffraction (XRD), and Scanning Electron Microscopy (SEM). The Friedman differential isoconversional method was applied for determining the effective activation energy for non-isothermal crystallization, since it gives relevant information without knowing the form of the kinetic equation. A modified Avrami model was used to calculate the n values; it was found that they are near 2.5, for all analyzed samples, which means that it is related to the crystallization mode diffusion controlled, with constant nucleation rate and three-dimensional growth. This agrees with the SEM micrographs, where dendritic structure is observed for all crystalline samples. Additionally, structural information got from Raman spectroscopy, for the samples in vitreous state, was used to interpret crystallization tendency, i.e., the fact that crystallization was enhanced by increasing Na₂O content, due to slag depolymerization. Moreover, computational thermodynamics was used to analyze mold fluxes crystallization behavior.

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Series: ISIJ international
ISSN: 0915-1559
ISSN-E: 1347-5460
ISSN-L: 0915-1559
Volume: 60
Issue: 11
Pages: 2425 - 2435
DOI: 10.2355/isijinternational.ISIJINT-2020-132
Type of Publication: A1 Journal article – refereed
Field of Science: 216 Materials engineering
Funding: Financial supports from the Academy of Finland for Genome of Steel Grant (No. 311934), from the National Natural Science Foundation of China (NSFC No. 51774026), and from the Coordination for the Improvement of Higher Education Personnel of Brazil (CAPES, Finance Code 001) are gratefully acknowledged.
Copyright information: © 2020 The Iron and Steel Institute of Japan. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs license.